The present invention relates to unsaturated elastomeric ABC block copolymers based on butadiene, isoprene and optionally styrene, their manufacture and their use for the manufacture of tire treads.
It is generally accepted that rubbers which are to be employed in tire making must satisfy the following requirements:
Cold creep must be as low as possible. PA1 The rubbers must be readily processable in subsequent blending processes. PA1 The rubbers must be flowable during the molding processes. PA1 The rubbers must be readily vulcanizable. PA1 (a) They are required to remain highly resilient even at low temperatures. PA1 (b) They must exhibit good anti-skid properties in wet conditions. PA1 (c) They are required to have high abrasion resistance to provide a correspondingly long life expectancy. PA1 (d) When subjected to dynamic loads they should generate as little heat as possible. Their rolling resistance is to be as low as possible in order to keep the fuel consumption of the vehicle as low as possible. PA1 1. The block copolymers do not satisfy adequately the above-mentioned requirements with a view to their use as tire materials. PA1 2. Compatibility problems of the two blocks are experienced. PA1 3. The tan delta curve exhibits only a narrow damping maximum. PA1 4. Large amounts of comparatively expensive isoprene are required. PA1 either butadiene and isoprene units with a vinyl content of less than about 15 wt%, PA1 or butadiene units having a vinyl content of more than about 75 wt%, and PA1 either butadiene and isoprene units with a vinyl content of 8 to 15 wt%, PA1 or butadiene units having a vinyl content of 75 to 90 wt%, and PA1 about 40 to 140, preferably 35 to 120; PA1 about 0.5 to 5.0, preferably 0.6 to 3.0; PA1 &gt;about 10, preferably &gt;20. PA1 ethers, PA1 tertiary amines or PA1 tertiary amines containing ether groups or mixtures thereof. PA1 the tetrahalogenides of the elements Si, Ge, Sn and Pb, in particular SiCl.sub.4, PA1 organic compounds of the general formula R.sub.n [SiHal.sub.3 ].sub.n, wherein n=1 to 6, in particular m=1 and 2. In this context R is an organic moiety bearing 6 to 16 carbon atoms and having a valency of n, for example an aliphatic, cycloaliphatic or aromatic moiety. 1,2,4-Tris(2-trichlorosilylethyl)-cyclohexane, 1,8-bis(trichlorosylyl)-octane and 1-(trichlorosilyl)-octane may serve as examples. PA1 Organic compounds which contain at least one of the moiety &gt;SiHal.sub.2, e.g., dimethylsilylchloride. PA1 Halogen hydrosilanes of the general formula Si(H).sub.m (Hal).sub.4-m wherein m is from 3 to 1 PA1 di- and trivinylbenzenes, e.g., 1,4-divinylbenzene. PA1 high-skid resistance under wet conditions, PA1 high abrasion resistance, PA1 low rolling resistance and thus low fuel consumption, PA1 high wear resistance, and PA1 all-weather suitability.
In addition, special requirements have to be complied with which arise from their particular application in tire making. It is well known that in recent times increased demands are being made on the properties of tire treads:
It is known that rubbers, when subjected to torsional vibration tests exhibit a temperature dependency of the logarithmic decrement of mechanical damping and derived therefrom a temperature dependency of the mechanical loss factor tan delta which, when expressed as a graph, yields a graph configuration which is characteristics for the particular rubber. The desired requirements for tire treads are met in particular if the tan delta-curve comprises a vibration damping range which is as wide as possible (cf. K. H. Nordsiek. Kautschuk und Gummi, Kunstoffe 38, 178 (1985) and 39, 599 (1986).
It is also known that these partly contradictory properties of tire treads are determined to a substantial extent by the nature and composition of the rubbers employed for this purpose.
The homopolymers based on the conventionally employed monomeric raw materials such as butadiene, isoprene and styrene do not meet these requirements satisfactorily (cf. EP-OS 0 054 204 and JP-OS 82/87 406).
Blends of rubber types are in practice subject to the disadvantage that the above stated spectrum of properties is not attained and the desired technological qualities for tires are not reproduced reliably. Accordingly, there exists a need for rubbers which substantially satisfy the aforesaid desired properties. In principle it should be possible to attain this object with rubbers composed of polymers comprising a variety of blocks.
For purposes of this invention, the meaning of blocks of a polymer is not restricted to only chain segments each composed of different monomeric building elements, but also includes those segments which--dictated by the extraneous process parameters--vary in their nature of interlinking of the monomeric building elements or in the proportion in which they are inserted in a chain segment. Although the butadiene-isoprene copolymer described in EP-OS 0 054 204 comprises in its initial and terminal portion a different content of isoprene as a result of the lower tendency of isoprene to polymerize as compared with butadiene, it is not to be considered a block copolymer within the meaning just explained.
Even if during the copolymerization of dienes and styrene the styrene proportion is changed (cf. DE-OS 31 08 583) no block copolymers are attained, but merely a gradual transition. The desired improvement of tire technological properties is still inadequate, even in that case. Single phase rubber systems are described in DE-OS 31 08 583 comprising a damping maximum created by a glass transition point in a very narrow temperature range.
An improvement is attained only by virtue of a copolymer being produced comprising two different blocks A and B which differ in their structure and/or composition.
A statistical styrene-butadiene block copolymer is thus described for example in DE-OS 31 51 139. The blocks differ in their butadiene contents and their contents of vinyl bonds. They are so intermixed that they are rendered compatible and that instead of two separated damping maxima only a single such maximum is created.
In DE-OS 35 30 438 rubber compositions are claimed which comprise at least 20 wt% of a styrene-butadiene block copolymer. The blocks differ with respect to their styrene contents, their vinyl bond contents and as a result thereof their glass transition temperatures. In that case as well, the tan delta curve exhibits only a narrow temperature range of maximum damping.
Japanese published specification 83/122 907 describes branched rubbers which may be obtained by the conversion, e.g., of a metallic tetrahalogen compound, such as SnCl.sub.4 with block copolymers comprising a polysioprene and a polybutadiene block. Thus each of the two blocks is present as a homopolymer. The starshaped rubber which is attained after conversion with the metallic coupling agent forms a single phase rubber system having a glass transition point.
GB-PS 2 090 840 describes block copolymers which are attained by the polymerization of dienes or the copolymerization of diene mixtures and the blocks of which differ in respect of their contents of 1,2 and/or 3,4 structural units by 20 to 50 mol wt%. The preparation of such block copolymers takes place in the presence of different amounts of cocatalyst or at different temperatures.
Tire treads are described in EP-OS 0 173 791, the rubber component of which may be composed to 30 to 100 wt% of block copolymers based on butadiene, isoprene and optionally sytrene and/or piperylene. The block copolymers are produced in the presence of cocatalysts by increasing the temperature and may comprise AB or ABC structures. The polymers always contain a terminal block based on butadiene which is formed at rising temperatures and which accordingly comprises a comparatively high content of 1,2 structural units and an uneven distribution of the vinyl groups. However, these block copolymers do not yield a tan delta-curve having an adequately wide plateau in order to comply optimally with all required tire properties (see comparative example A). Accordingly, even that sspecification proposes to blend the block copolymers so obtained with other rubber components (see claim 1 and example 2). All aforementioned block copolymers are subject to at least one of the following shortcomings: